1889-90.] On the Echinoidea or Sea-UrcJnns. 351 



kingdom. The modus <yperancli of the ambulacra is well 

 described by Dr Eomanes in the work already referred to. 

 In the animals which compose the group, the functions of the 

 tube-feet are found to be developed in an inverse ratio to 

 the functions of the rays. That is to say, in the sea-urchin, 

 where the rays are fixed, the tube-feet are most highly de- 

 veloped ; in the Brittle-stars, where the rays are extremely 

 mobile, permitting these creatures to indulge in all sorts of 

 acrobatic feats impossible to a sea-urchin, the tube-feet are 

 mostly rudimentary ; while between these two extremes are 

 the common star -fishes, where the rays are beginning to 

 assume a mobile character, and the tube-feet are consequently 

 neither so long nor so powerful as in an echinus. The tube- 

 feet of a sea-urchin are extremely serviceable as anchors ; 

 and when we remember the globular shape of the animal, 

 and how it is very much at the mercy of waves and currents, 

 we can easily understand how such an anchoring apparatus 

 is necessary. So firmly does it hold on by the tube-feet, 

 when the attachment is properly formed, that it will permit 

 the sucker-discs to be torn away rather than let go its hold. 

 This a star-fish will never do, but will permit itself to be 

 tossed to and fro by the waves, knowing by instinct, so to 

 speak, that it can easily right itself again by means of its rays. 

 Besides the powerful muscular rings present in the tube-foot 

 of an echinus, the tube-walls are strengthened by numerous 

 spicules, shaped like the letter C. The suctorial disc, again, 

 is composed of plates with waved margins, from five to eight 

 in number, which form interesting micro -preparations, and 

 can easily be dissolved out in caustic potash. Prof. Forbes 

 calculated that a full-sized sea-urchin would carry 1860 tube- 

 feet. 



As to the internal structure of a sea-urchin, the masticatory 

 or dental apparatus first claims attention. The pentamerous 

 arrangement already observed in the structure of the shell 

 again appears here, for the masticatory apparatus is made up, 

 in all, of twenty pieces, arranged thus : Five sharp-pointed 

 triangular teeth ; five sheaths or alveoli, in which the teeth 

 are laid, and in which they move freely iip and down by 

 means of powerful muscles ; five wheel-pieces or rotulae, which 

 stretch from one sheath to the next, and form a circle ; and 



